The structure of yeast nuclear pore complexes is being studied both by cryo-electron microscopy and 3-') image reconstruction and jhe STEM. STEM mass analysis of the complexes is being used to place volume constraints on 3-') maps being calculated using the random conical tilt method. The mass analysis will also place firm limits for fliture work on the stoichiometry of nucleoporins. Preliminary mass measurements gave an estimate of 65-68 Md for an intact complex, but clearly some specimen disassembly was occurring during specimen preparation. Improved specimens have given much better mass measurements which has permitted good 3-') image reconstruction.